Pour into a greased 9x13 pan and bake using the baking instructions on the box.
If making homemade whipped cream: In a large mixing bowl, combine the heavy whipping cream, powdered sugar, and vanilla extract. Using a mixer, whip the cream mixture on medium-high speed until stiff peaks form.
Once the cakes are completely cooled, cut the 9x13 into two layers down the long side. Place one layer on a serving plate or cake stand. Spread a generous layer of whipped cream over the top of the bottom cake layer. Arrange a layer of sliced strawberries over the whipped cream. Place the second cake layer on top and gently press down to secure.
Frost the top and sides of the cake with the remaining whipped cream. Garnish with additional sliced strawberries and whole strawberries if desired.
Refrigerate the cake for at least 1 hour before serving.
Notes
Optional: You can also make this recipe using 2 round cake pans instead of a 9x13.